Thrustmeter



L. A. WEISS Sept. 26, 1961 THRUSTMETER Filed Oct. 26, 1954 INVENTOR.

LEO A. WEIS S 3,001,716 THRUSTMETER Leo A. Weiss, Kings Point, N.Y.,assignor to Avien, Inc,

Woodside, N.Y. Filed Oct. 26, 1954, Ser. No. 464,685 1 Claim. (Cl;235-193) This invention relates to analogue computers and in" particulara thrustmeter for, measuring the thrust of a jet engine.

In the operation of jet powered aircraft, it is essential that the pilothave accurate information of the thrust of K =a constant depending onthe particular engine, I

=ratio of specific heat under constant pressure to that under constantvolume of the jet exhaust gases.

A=area of exhaust nozzle in square inches.'

P =absolute ambient pressure in p.s.i.

P =absolute pressure in the tailpipe in p.s.i.

The value of 'y is nearly constant, and equal to 1.33.

Then

F 8 06K AP m 1 F r ll?) A pair of transducers are employed to measureabsolute ambient and tailpipe pressure, i.e., P and P Equip ment is,therefore, required which will interpret the output of the transducersin terms of thrust in accordance with the above equation. An analoguecomputer for accomplishing this computation which-would be practical toinstall and operate aboard a jet aircraft must meet stringentrequirements including space, weight, stability and vibrationrequirements. A computer which meets these requirements is disclosedhereinafter.

It is an object of this invention to provide a compact analoguecomputer.

It is another object of puter capable of determining variables raised toa power.

It is a particular object of this invention to provide an improvedthrust measuring apparatus.

Another object is to provide a' pressure measuring circuit controlling ashaft whose change in angular position is directly proportional tocorresponding changes in the pressure under measurement.

Another object ofrthis mvention is to provide'a measuring device havingindependent adjustments for maximum and values of the condition to bemeasured. T

An object of this invention is to provide a thrustmeter havingindependent adjustments for minimum and maximum pressures to bemeasured.

Other objects and advantages will become apparent from consideration ofthe following discussion taken in conjunction with the accompanyingdrawings.

In the drawing there is shown schematically the apparatus of thisinvention.

In the drawing there is shown a bridge circuit 2 of the self-balancingtype used for the measurement of pressure in conjunction with acapacitor type of transducer sensing the said pressure.

its output shaft rotation,

this invention to provide a comfunctions of a plurality of 6, isdirectly proportional to A feature of this bridge is that;

Patented Sept. 26, 1961 "ice consisting of two capacitances C, and CCapacitance C is a fixed reference capacitor 8 while 0,, represents thecapacitance of transducer 10. The voltages e, and e, are obtained from atransformer 12 with its primary winding 14 connected to a source ofalternating power. The voltage a in series with the transducer 10 isvariable, and is obtained from the secondary winding 16 of thetransformer through an adjusting or calibration potentiometer .18 and arebalancing potentiometer 22. The voltage e in series with the referencecapacitor 8 is obtained through a calibration potentiometer 20. A motor26 sensitive to polarity or phase is mechanically coupled through asuitable device, such as a gear train, or a worm drive, to the Wiper 24of the rebalancing potentiometer 22. Thus, the angular position 0 of thepotentiometer wiper 24 determines the value of the voltage e Since theshaft 25 of motor 26 is coupled to the wiper 24 of the rebalancingpotentiometer 22, it follows that the voltage e, is controlled by theangular position of shaft 25 of motor 26. I

An amplifier 30 of the electronic or electromagnetic type is connectedto the signal terminals 32 and 34 of the bridge, across which a voltages may appear. In general, the bridge is considered to be balanced when eis zero. If the bridge is not balanced, the small voltage e is amplifiedby the amplifier 30 and fed into the motor 26. The polarity of thevoltage e is such that the motor 26 will rotate the wiper 24 of therebalancing potentiometer 22 in a direction that will change e, to avalue required for balance.

The balance conditions of the bridge require that Further, thecapacitance c of the transducer is inversely proportional to theabsolute pressure P, which is to be measured, letting OFF-KI? where K,is a constant of proportionality depending on the geometry and thephysical constants of the transducer, d is the spacing between thetransducer plates at zero pressure, and K, is the deflection constant ofthe transducer bellows, the substitution of C, as given by Equation 2 inEquation 1 provides Equation 3 as follows:

Equation 3 may be rewritten for fl=et max in which case -P==0. Then e adwhere 0 max is the total angular sweep of the wiper 2.4, and e, m is thevalue of e, when the wiper 24 is at the lowest end of the rebalancingpotentiometer 22,.

four arm bridge, two adjacent arms 3 It follows from 5 and 6 that mnz rI d P: max rd P t( t max t min) K t max min) depend only on the turnsratio of the transformer 12 and not on the supply voltage. Hence, if thecalibrating potentiometer 18 is properly set, so that the Wiper 24 is atthe end 36 of the potentiometer 22 when the transducer is exposed toZero absolute pressure (i.e. vacuum), the angular position 0 of thewiper 24 is proportional to e, and, in accordance with Equation 3, 0 isproportional to the pressure P, under measurement.

The calibrating potentiometer 18 adjusts the minimum value of thepotential a when the wiper 24 is at the end 38 of the rebalancingpotentiometer 22. It should be noted that the maximum value of e,,obtained when the wiper 24 is at the position 36 of the rebalancingpotentiometer, is independent of the setting of the calibratingpotentiometer 20, so that the adjustment of the calibratingpotentiometer 20 does not affect the setting of the calibratingpotentiometer 18 as mentioned above.

Rotation of motor 26 actuates both shafts 25 and 40 so that the angularposition of 40 is also proportional to the pressure P under measurement.

Bridge circuit 2 may be used to detect the absolute pressure in the tailpipe of a jet exhaust whilea similar circuit 42 is used to measure theambient pressure. The angular position of shaft 44 will accordingly beproportional to the ambient pressure P,,.

Thus there is available two shafts whose angular positions represent thepressures P and P A Wheatstone bridge circuit 50 is used as an analoguecomputor to calculate the ratio between P the absolute pressure in thetail pipe, and P,,, the absolute ambient pressure. Wheatstonebridgecircuit 50 is composed of four arms 52, 54, 55, and 58 in series.The bridge is energized by transformer 60 connected between an AC. powersource and points 62 and 64. The bridge is provided with a pair ofoutput terminals 66 and 68 across which is connected amplifier -70.

Arm 52 consists of a linear variable resistance element 72, theresistance of which varies in accordance with position of arm 74. Theposition of the arm is, in turn, controlled by shaft 40. Accordingly,the resistance of arm 52 of the bridge is proportional to the tail pipepressure P Likewise arm 54 contains variable resistor 75 whoseresistance is controlled by movable arm 76 which is, in turn, actuatedby shaft 44 so that the resistance of arm 54 of bridge 60 isproportional to the ambient pressure P,,. Arm 55 is provided with fixedresistor 56 and an adjustable resistance element 78 which is used toobtain an adjustment for maximum ratio. Arm 58 includes an adjustableresistance element 82 which is provided with a movable arm 84 controlledby motor 86. Motor 86, in turn, is under the influence of amplifier 70and will operate only when a signal is applied by amplifier 70, -i.e.,'when the bridge 50 is not balanced.

Thus by varying the resistance of element 82, the bridge 50 may beelectrically balanced, at which time motor 86 is at rest. In addition toactuating arm 84, motor 86 actuates shaft 88. To adjust bridge 50movable arms 74 and 76 are positioned to simulate a high ratio of P to PThen rheostat 78 in arm 56 is varied until movable arm 84 moves to oneend of rheostat 82. For adjustment of the bridge for minimum ratioreadings, movable arms 74 and 76 are positioned to stimulate a low P toP ratio. Then rheostat in arm 58 is varied until movable arm 84 moves tothe other end of rheostat 82. Alternate adjustment of rheostats 80 and78 will have to be effected until the bridge is balanced. The angularposition of shaft .88 is proportional to the ratio t a The output ofbridge 50, namely: the ratio t .P in terms of angular shaft position, isthen introduced to a second self-balancing Wheatstone bridge circuit 92.

Bridge 92 is provided with four series connected arms 94, 96, 98, and100.

Bearing in mind that the equation to be solved is we may now considerthe circuit of this second Wheatstone bridge.

Let

R the resistance of arm 94 R96 the resistance of arm 96' R98, theresistance of arm 98 R the resistance of am 100 then in accordance withthe relationship between arms of a Wheatstone bridge r R 94)( .9s) 00 fArm 94 includes a variable rheostat having an element wound or shaped soas to produce an output which varies with change inangular position ofshaft 88 accord ing to the function:

Am 96 includes rheostat 101 which may be preset to provide the'constantmultiplying term, 8.06 K A.

Thus, the finally adjusted resistance of element 110 corresponds to Fthe thrust.

The value, 8.06 K A is fixed by the value of the resistance of arm 96.Shaft 107 is coupled to motor 26a and varies the position of the wiper109 of rheostat 108 to introduce the P term.

The bridge is energized from an AC. source through transformer 114 whichis connected to the bridge at junctions 110 and '1-16. v r

The outputof bridge circuit :92 is detected across juncamplifieroperates motor 126 which,'through shaft 127,

adjustsvafiable .arm .112 of rheostat 110to adjust the re sistance ofarm .so as 'tobalance'the bridge. Motor 126 through shaft 129 alsovaries the posit-ion of pointer 128 .of indicator .130. the position ofpointer 128 is proportional to the thrust, wF

A pressure sensingelement suitable for use in the ap paratus of thisinvention is disclosed in the .copending application of Martin Ruderferentitled, Transducer, filed concurrently herewith, which issued asPatent Number 2,714,703 on August .2, 19.55 and assigned to the assignee.ofthe present invention.

The formula given earlier for thrust, i.e.

is presently considered by experts in the field as accurate. It is to beunderstood, however, that the usefulness of this invention is notnecessarily dependent upon the correctness of this formula, nor is itnecessarily limited to the measurement of thrust. The invention appliedequally well to any formula which can be expressed exactly or withreasonable approximation in the same mathematical form.

The voltage responsive means controlling the motors may be an amplifierof the electron tube, transistor or magnetic types.

While I have discussed the application of the invention to a jet exhaustengine, be it understood that this invention applies to devices forcontinuously measuring and indieating the thrust of other reactionengines such as ram jets, turbo jets, rockets, guided missiles and thelike.

While I have disclosed the best mode contemplated of carrying out theinvention, be it understood that further modification and changes may bemade within the scope of the appended claims.

What is claimed is:

Computer means for providing an indication related to a first and asecond condition to be sensed comprising: a first variable capacitorarranged to be varied in accordance with the said first condition to besensed; a second variable capacitor arranged to be varied in accordancewith the said second condition to be sensed; a first selfbalancing meansresponsive to said first condition to be sensed including, a fixedcapacitor, first and second sources of voltage, one of which is fixedduring such normal measuring operation and the other of which isvariable in accordance with said first condition to be sensed, a circuitarranged to be connected to said first variable capacitor and one ofsaid sources of voltage and a further circuit including said fixedcapacitor and the other of said sources of voltage, said circuits havinga common impedance with respect to which said voltage sources are inopposition so that when the currents through the two circuits are thesame no voltage appears across the said common impedance portion, avoltage responsive motor controlling means connected across said commonimpedance portion of said circuit, a motor controlled by said motorcontrolling means, means positioned by said motor to adjust saidvariable source of voltage in a direction to reduce the voltageimpressed on said voltage responsive means until the latter isineffective to cause operation of said motor; a second self-balancingmeans responsive to said second condition to be sensed including a fixedcapacitor, first and second sources of voltage, one of which is fixedand the other of which is variable in accordance with the condition tobe sensed, a circuit including said second variable capacitor and one ofsaid sources of voltage and a further circuit including said fixedcapacitor unit and the other said sources of voltage, said circuitshaving a common impedance portion with respect to which said voltagesources are in opposition so that when the currents through the twocircuits are the same no voltage appears across said commonimpedance-portion, a voltage responsive motor controlling meansconnected across said common impedance portion of said circuit, a motorcontrolled by said motor controlling means, means positioned by saidmotor to adjust said variable source of voltage in a direction to reducethe voltage impressed on said voltage responsive means until the latteris ineiiective to cause operation of said motor; a ratio bridge having afirst pair of terminals across which a potential is impressed and aconjugate pair of terminals across which an output voltage is taken,means to vary the output voltage between said conjugate pair ofterminals comprising a variable resistance device under the control ofsaid first self-balancing means motor and in series therewith, a secondvariable resistance device under the control of said secondself-balancing means motor, a third variable resistance device having amovable tap, a fourth resistance device in series with said thirdvariable resistance device, said third and fourth resistance devicesbeing in series with said first pair of terminals, a voltage responsivemotor controlling means connected across said ratio bridge outputterminals, a motor controlled by said motor controlling means connectedin operative relation with said movable tap to move said tap in adirection to reduce the said ratio bridge output voltage until thelatter is ineffective to cause operation of said motor; a product bridgecomprising a four arm Wheatstone bridge circuit in which one armincludes a variable resistance which is varied by said secondself-balancing means motor, a second arm including a variable resistorwhich is varied by said ratio bridge rebalancing motor, a source ofoperating voltage applied to one pair of diagonal terminals of thebridge, voltage responsive means connected to the conjugate terminals ofthe bridge responsive to a difierence in voltage thereacross, a motorcontrolled by said voltage responsive means, a rebalancing third armincluding a variable resistor having a movable arm positioned by saidmotor to adjust said variable resistance in a direction to reduce thevoltage impressed on said voltage responsive means until the latter isineffective to cause operation of said product bridge motor, a fourthresistance arm in series with said third arm; and indicating meanspositioned by said product bridge motor.

References Cited in the file of this patent UNITED STATES PATENTS1,706,968 Schleicher Mar. 26, 1929 2,349,860 Hainer May 30, 19442,431,696 Keister Dec. 2, 1947 2,453,607 Whardle et al Nov. 9, 19482,539,616 Gehman Jan. 30, 1951 2,579,617 Schaevitz Dec. 25, 19512,584,897 Marco Feb. 5, 1952 FOREIGN PATENTS 921,205 France J an. 10,1947

